Thanks! I hadn't thought of using setImmediate.
That means my transform stream constructor would become a single call to
setImmediate, which would then do all the work. Because it's not errors from
`spawn` so much as it's errors I'd like to emit from the constructor in
response to incorrect parameters before I even call `spawn`.
It sounds like you would say that in a stream I should emit errors, never throw
them?
On Jun 17, 2013, at 20:35, Dan Milon wrote:
> You can wrap `spawn` in a `setImmediate`, so that others get a chance to
> attach their events.
>
> On 06/18/2013 04:19 AM, Ryan Schmidt wrote:
>> Per previous threads, I'm implementing a transform stream around a
>> particular command line program for use in a web service. How should the
>> stream report errors?
>>
>> In the case of the _transform and _flush functions that I'm supposed to
>> implement, they're given a callback parameter, so I can "callback(new
>> Error(…))". That's easy, but what about in other situations, like the
>> constructor?
>>
>> In the constructor of my custom transform stream, I spawn a process:
>>
>> var process = this._process = spawn(options.command, options.args,
>> options.env);
>>
>> If the process closes with a non-zero exit code or crashes, I want to be
>> able to report that, not just to the server's console, but also to the web
>> browser of the user who made the request that started the process:
>>
>> process.on('close', function(code, signal) {
>> if (signal !== null) {
>> …
>> } else if (code !== 0) {
>> …
>> }
>> self.emit('close');
>> });
>>
>> If a problem occurred, should I "throw new Error(…)" or "self.emit('error',
>> new Error(…)"?
>>
>> I was emitting the error, but am having trouble making that work in code
>> running directly in the constructor. Let's say I want to allow only a few
>> different commands ("foo" and "bar") to be run:
>>
>> if (!~['foo', 'bar'].indexOf(options.command)) {
>> this.emit('error', new Error('Invalid command: ' + options.command));
>> }
>>
>> If I emit an error like that, then the node program exits saying there was
>> no listener attached to the stream listening for the error event. But I
>> cannot attach a listener to the stream until after the constructor has
>> finished and has returned the stream object to me.
>>
>> Should I instead throw the error? If so, do I just use a try{}catch{} block
>> whenever I create such a stream?
>>
>>
>> Passing Error objects to a callback is straightforward, but other methods of
>> error handling in node are still very confusing to me, especially try/catch.
>> Guidance would be appreciated.
>>
>>
>
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